I started commenting on appropriate CYA levels in an SWG pool in the CYA / Pool Pilot Question thread and I was pointed to this thread which has been very informative (and has me eat crow).

Obviously with my technical interest I would like to get to the bottom of why higher CYA levels are needed in an SWG pool. Assuming the data on the breakdown of free unbound chlorine vs. chlorine tied up in CYA is correct, then the amount of extra work (time) the SWG needs to be on at 40 ppm CYA is only 12% higher than at 80 ppm CYA with 3 ppm FC (it's a 45% increase in work or time going from 80 to 20 ppm CYA). Since most people see a much bigger jump than that (to maintain the same FC level), either the chlorine breakdown data from sunlight is wrong or CYA is needed in the SWG process to improve its efficiency.

It may very well be that what was said earlier on this thread is what is happening. Namely, that rather high localized concentrations of chlorine (HOCl, OCl- from dissolved Cl2) are generated and that with CYA these high concentrations get tied up quickly before they have a chance to breakdown. I find this somewhat surprising as a decent flow rate should dilute the concentrations rather quickly, but physical flow rates are usually not nearly as fast as most chemical processes (more on that later in this post).

The best way to test this theory would be with a pool that is covered to minimize the breakdown from sunlight to remove that factor from the experiment (also try not to use the pool during the experiment). Since the chlorine demand would be lowered, the chlorine generator cell should be set to run less frequently (lower percentage) to maintain a constant FC level, but the cell should be run at the normal power level used in a typical uncovered pool. Start out with a CYA level of 40 ppm or below -- way below, such as 20 ppm, would be even better. Record what percentage of time the cell needs to be on to maintain a constant FC level (say, 3 ppm). Then add CYA to the recommended range of 80 ppm. Adjust and record the percentage of time the cell needs to be on to maintain a constant FC level. Report back to this thread your results.

The above experiment was sort of done by some members of this forum (and posters to this thread) but I don't believe a cover was used so the variability in sunlight is a problem since this is a huge factor in chlorine breakdown.

I am still puzzled by the prevention of algae in an 80 ppm CYA 3 ppm chlorine pool since that only produces 0.015 ppm HOCl at a pH of 7.5 which is enough for disinfection, but presumably not enough for preventing algae (in Ben's experience). Though some have said that the higher chlorine concentrations in the SWG cell may kill algae, that would not be true for any algae that adhered to the plaster. So maybe the SWG cell kills free-floating algae and that usually that is good enough.

A post from "chem geek" would be incomplete without some technical analysis to scare the bejesus out of 90% of the forum, so stop reading at this point if you're (rightfully) frightened by chemistry.

I actually have the rate constants for the conversion of HOCl into the chlorinated cyanurates (i.e. the take-up by CYA). My rough calculations show two things. First, that a level of CYA of around 50 ppm may be the concentration where the rate of take-up by CYA of incrementally generated chlorine in the cell is equal to the generation rate of this chlorine. This lends support to the theory that the CYA helps the chlorine generation by preventing the buildup of unbound chlorine (i.e. HOCl and OCl-). Second, the incremental concentration generated by the cell is rather small so this build-up theory only works if there is a very high local concentration at the generation site (the electric plate) and that is certainly plausible. However, this whole business about superchlorination is a bit overblown since the calculations show that it would take nearly 100 turnovers of the pool water to shock it with the equivalent of 10 ppm. On the other hand, a shock of 1 ppm takes only 10 turnovers and 0.1 ppm only one. Therefore I don't believe the theory of the "supershock" is the appropriate one because not enough water is exposed to 10 ppm or higher levels of chlorine, but free-floating algae even with Ben's chart is prevented by 0.05 ppm HOCl.

Richard